The present invention relates to filter circuits for processing digital signals.
FIG. 2 shows a constitution of a transversal filter most used as a digital filter. The transversal filter in FIG. 2 comprises a device for delaying a signal, represented by a shift register 11, a plurality of multipliers 31 for, multiplying output signals of respective stages of the shift register 11 being different in a delay time (in FIG. 2, Z.sup.-1 indicates a unit delay time) by tap gains C.sub.0, C.sub.1, C.sub.2, . . . , C.sub.n respectively corresponding to the stages of the shift register 11, and an adder 41 for obtaining a sum of outputs of the multipliers 31. The plurality of multipliers 31 are put together and represented as a multiplier group 310. Besides this, various digital filters are known to perform signal processing by a combination of delay, multiplication and addition. In the factoring description, such a process of delay, multiplication and addition to input signals performed by the digital filter is called a filtering process.
A sampling frequency fs at which a video signal is sampled, for example, in the case of the NTSC system, is 14.3 MHz being four times of a color subcarrier frequency fsc. Then, a sampling interval (=1/fs) becomes 70 ns. That is, when the above-mentioned transversal filter is constituted in the video signal band, a next signal appears every 70 ns at each stage of the shift register 11.
On the other hand, with the development of semiconductor circuit technique, computing elements such as an adder, a multiplier or the like have been significantly improved in an operation speed. For example, a multiplier of 8 bits.times.8 bits in 8 ns can be realized.
The transversal filter in FIG. 2 is provided with one multiplier for each tap. If the above-mentioned filter is used, it takes only 8 ns from the determination of signals of each stage of the shift register 11 to the output of the multiplication result, and since all multipliers are not operated during a period of 62 ns until a next signal appears in each stage, such a transversal filter is not efficient.
To solve this problem, recursive operation of the computing element as disclosed in the Japanese Patent Application Laid-open No. 63-1258 has been proposed. The computing element is operated at the frequency of n times the sampling frequency of the input signal, thereby the operation for n data will be processed by one computing element for one sampling interval. The scale of the circuit would be reduced because of replacement of n computing elements by one computing element.